In the power plant, steam generated in, for embodiment, an atomic reactor (steam generator) drives a turbine, is condensed by a condenser, and water thus condensed is supplied to the atomic reactor via a pump and a feedwater control valve. In such a system, an amount of feedwater to the atomic reactor, which is a steam generator, is controlled by inputting an atomic reactor level signal, a main steam flow signal and a feedwater flow signal into a feedwater controller, by adding, in the feedwater controller, a deviation signal between steam flow rate and feedwater flow rate to a deviation signal between the atomic reactor level and the set point for carrying out a PI operation, and by controlling an opening of the feedwater control valve on the basis of an output signal from this feedwater controller. Such a feedwater control system has been described in, for embodiment, JP-A No. 33002/1983.